The present invention relates to impact printers and, more particularly, to a method of controlling impact printer noise.
A typical test system for evaluating radiated acoustic noise problems comprises a forcing function, whose content is characterized in the frequency domain by a spectral signature, a transfer function that modifies the spectral content of the forcing function, and a radiator that is driven by the output of the forcing function. The radiator converts the mechanical energy to acoustic energy. The final frequency content of the output acoustic energy impinging upon the ear of an operator is a result of the radiator dynamics and acoustic impedance of the medium through which the sound pressure waves travel.
Noise control of acoustic energy heard by the human ear can be accomplished by modifying one of the aforementioned three test system elements.
In real systems comprising retail and financial impact printers, the total acoustic noise generating system is more complex than described hereinabove. Elements of printer acoustic systems have multiple force inputs, such as stepper motors, print heads, and solenoids. Many transfer functions are defined by platens, carriage shafts, mechanical connections, etc. These transfer functional components create a multiplicity of cross coupled, vibration pathways, through which mechanical vibrations can reach the radiators. The primary radiators of the commercial impact printers manufactured by the present assignee usually become the cabinet and the receipt, journal, or slip paper that is printed. In assignee""s printer systems, control of acoustic noise can be accomplished by making changes to the matrix of any one of the aforesaid system elements.
Unfortunately, noise control techniques are usually applied to these commercial printers after the fact. This results in expensive and unsuitable methods of damping or attenuating acoustic borne noise. The acoustic noise leaks out of the openings in the cabinet or emanates directly off of the cabinet surfaces.
Other noise reducing methods have been tried that modify the transfer functions involved in the system. Typically, these include rubber isolators, massive platens, or appropriately designed and placed damping materials. These solutions tend to be expensive, ineffective, and undesirable.
The present inventors of assignee, Axiohm Corporation of Ithaca, N.Y., have discovered that modifying basic forcing function inputs results in more effective control of the final printer acoustic noise output.
In assignee""s impact printers, such as model nos. 7150, 7221, and A758, the primary system input forcing function comprises the impact print head wires that strike the platen. Time domain force cell measurements have shown the shape of the input forcing function is triangular or saw toothed. In actual operation, the forcing function becomes repetitive at 900 to 1000 Hz. Amplitude of this function depends on the number of dots being fired during a single impact time.
This waveform can be spectrally viewed as the convolution of a single finite energy triangular waveform with a comb function. The impulses of the waveform are separated in the time domain by the repetition rate period (trep). The resulting spectrum creates an envelope defined by the finite energy triangular waveform with harmonics inside that envelope spaced at 1/trep.
In scrutinizing this spectral view, it can be concluded that changing the shape or nature of the basic print head force input waveform can affect the entire acoustic output of the printer system.
Previous print head control electronics were limited in their ability to wave shape the input voltage pulses due to the inability of the main control processor to perform this function. Also, the cost of implementing specific electronic hardware circuits to perform this function was prohibitive. Recently, however, the cost of higher performance microcontrollers has been reduced.
Also, the invention reflects the fact that printer controllers can include Complex Programmable Logic Devices (CPLDS) or Field Programmable Gate Arrays (FPGAS), wherein impact force wave shaping can be accomplished more easily.
In accordance with the present invention, there is provided a method of controlling impact printer noise. The method changes the shape or nature of the basic print head force input waveform that affects the entire acoustic output of the printer system. Previous print head control electronics were limited in their ability to wave shape the input voltage pulses due to the inability of the main control processor to perform this function. Also, the cost of implementing specific electronic hardware circuits to perform this function was prohibitive. The method of the invention modulates the input pulse to the power amplifier. The resulting current waveform is then applied to the print wire solenoids. The final shape of the force input to the platen reduces or spreads the spectral envelope containing problem harmonics and acoustic output noise.
It is an object of this invention to provide an improved method of controlling impact printer noise.
It is another object of the invention to provide a method of modulating and/or shaping the input voltage and solenoid winding current of an impact printer in order to modify the impact force.